1. Technical Field
The present invention relates generally to color image reproduction apparatus including enlarging photographic printers and the like and, more particularly, to improvements in additive and subtractive color light sources for use in such color image reproduction apparatus.
2. Background Art
In state-of-the-art enlarging photographic printers, such as the Kodak Create-A-Print Enlarging Minilab, an additive light source provides "white" light for both a scanning and a printing operation. The additive light source contains three lamps each providing light of a different primary color (i.e., red, green and blue). The primary color light components of the lamps are combined by a light integrating chamber to produce a white light.
For printing, it is necessary to balance the white light to compensate for (1) the transmittance characteristics of a photographic negative to be printed, and (2) the spectral sensitivity response of the printing paper used in the photographic printer. The white light is balanced by adjusting the intensities of the lamp outputs. This is accomplished mechanically by placing rotatable, metal perforated, attenuator wheels in the beam path of each lamp.
An exposure determination system, forming part of the printer, derives the correct angular positions of the attenuator wheels to introduce the proper amount of attenuation in the beam path of each lamp. The white light that results, is properly balanced to compensate for both the negative and printing paper. The exposure determination system includes a video camera having a solid-state, charged coupled device (CCD) type imager, an exposure control computer, and a video monitor for displaying the image to be printed.
During the scanning operation, the video camera receives a projection of an image contained on the negative, as produced by the white light passing through the negative. The camera converts the received image projection to R,G,B color video signals The signals are then prepared for processing in the exposure control computer. The computer compares the color signal information with stored calibration data and, from this comparison, it determines the transmittance characteristics of the negative The transmittance data is used by the computer to determine exposure parameters such as classification, subject failure suppression, slope correction, and attenuator wheel positions.
Due to the spectral sensitivity response of the CCD imager in the video camera, errors in determining the transmittance characteristics of the photographic negative are introduced. It was discovered that these errors could only be eliminated by compensating for the spectral sensitivity response of the video camera (as established by the CCD imager).
One approach to compensating for the video camera is to adjust the gains in certain amplifier circuits used in the exposure determination system. This approach had a substantial drawback in that it introduced excessive noise into the system, causing additional errors due to a low signal-to-noise ratio. Another approach is to adjust the balance of the white light using the attenuator wheels in the additive light source. This approach imposed a severe limitation on the range of attenuator wheel positions available for compensation of the negative.
A further compensation approach is to introduce a beam splitter in the optical path of the projected negative image. The beam splitter contains a yellow, partially silvered, reflective coating that enhanced the blue component of the projection being received by the camera. This approach had a disadvantage in that the coating on the beam splitter introduced "color stripe" artifacts on the printing paper during the printing operation.
In addition, the overall intensity of the projected light being received by the video camera and the printing lens is reduced by beam splitting. As a result, the depth-of-field of the video camera is limited, because the camera lens iris must be opened beyond the range of desired aperture settings to compensate for the reduced light. A limitation on the depth of field, in turn, results in a limitation on the sharpness of the image displayed on the video monitor Further, the ability of the printer to scan over-exposed negatives is degraded due to a decrease in light level.
In the original Kodak Create-A-Print Enlarging Minilab, all of the above-mentioned compensation approaches were employed together, with the objective that the drawbacks of each approach could be minimized. This objective was realized. However, these compensation approaches continued to limit the overall system performance of the printer to a degree that was not acceptable to the Eastman Kodak Company.
As indicated, during the printing operation, the attenuator wheels were employed to compensate for both the transmittance of the negative and the spectral sensitivity of the printing paper. This dual compensation role limited the range and resolution of the attenuator adjustments available from the attenuator wheels. Accordingly, it is desirable to have the attenuator wheels devoted to compensating for the photographic negative, and employ some other compensation approach for the printing paper.